1. Field of the Invention
The present invention relates to backlights for liquid crystal display devices.
2. Description of the Related Art
FIG. 27 is a cross section of a conventional backlight for a liquid crystal display device (see for example Liquid Crystal Display Technology. edited by Shoichi Matsumoto, Sangyo Tosho, p. 255). As shown in FIG. 27, the backlight includes a cold cathode fluorescent lamp (CCFL) 101, a reflecting mirror 102, a light guide plate 103, a reflecting white dot pattern 110 having scattering dot elements regularly arranged to radially scatter light, a reflecting sheet 107, a prism sheet 111, and a scattering sheet 112.
The backlight operates as described below: CCFL 101 emits light, which is directly or via reflection provided by reflecting mirror 102 incident on light guide plate 103 via a receiving end plane 120 and propagates through the light guide plate in repeated total reflection provided by a bottom plane 122 and an output plane 121 before the light emanates from output plane 121. When white scattering dot 110, provided in bottom plane 122 of the light guide plate, receives light, the light scatters, passes through bottom plane 122 and thus emanates outside light guide plate 103. The light emanating from bottom plane 122 is reflected by reflecting sheet 7 and is thus incident on bottom plane 122 and repeats the total reflection described above before it emanates from output plane 121. The light emanating from the light guide plate has its distribution adjusted by one or two sheets of prism sheet 111 and scattering sheet 112 for eliminating a small uneven distribution of light and irradiates a liquid crystal panel (not shown) arranged in front of the backlight.
Such a backlight uses scattering dot 110 to extract light. As such, the exact light emanating from the light guide plate has a distribution having a large angle and it thus does not always have a direction in which it is visually recognized. As such, prism sheet 111 is required to effectively collect light to allow the light to have an adjusted distribution collected in the direction in which the light is visually recognized. As such, one or two sheets of expensive prism sheet 111 are required, as described above, which increases the number of members of the backlight and complicates the fabrication process.
In order to minimize the number of members of a backlight, there has been proposed a guide light plate having an optical prism integrated thereto, as shown in FIG. 28 (Japanese Patent Laying-Open No. 10-282342). With reference to FIG. 28, among the members as described above, light guide plate 103 includes a reflecting, grooved portion 104 extending in a direction in which the receiving end plane extends, corresponding to a hollow space in the form of a wedge extending in the direction of a depth of the light guide plate, and an uneven portion 106 in the form of a prism having a plurality of parallel ridges extending in a direction orthogonal to receiving end plane 120.
Light incident on light guide plate 103 via receiving end plane 120 repeats total reflection as it propagates through the light guide plate. When a reflecting hollowed portion receives light on its oblique plane closer to the receiving end plane it reflects the light upwards. The reflected light does not have the conditions for total reflection and thus emanates from output plane 121 without total reflection. Output plane 121 has an uneven portion in the form of a prism to provide a narrow distribution of light. As such, a prism sheet can be dispensed with in controlling a distribution of emanating light. In such a configuration as above, a hollowed portion in the form of a wedge is required to have a shape controlled to correspond to its position in the light guide plate to provide a uniform distribution in density of emanating light over different points in the light guide plate.
Since the conventional backlight is configured as above, it requires a large number of independent members to allow emanating light to have an appropriate distribution. As such it is costly. Furthermore, the backlight using the conventional light guide plate having an optical prism integrated therein has a reflecting hollowed portion in the form of a wedge regularly arranged and it thus hardly provides emanating light with a spatially uniform intensity.
The present invention contemplates a highly efficient, low-cost backlight capable of providing appropriate light distribution and providing control to allow emanating light to have a spatially uniform intensity.
The present invention provides a backlight for a liquid crystal display device including a rod-shaped light source supplying light for light radiation for displaying information on a liquid crystal panel and a flat light guide plate having a side receiving end surface parallel to the rod-shaped light source, an output surface arranged opposite and parallel to the liquid crystal panel, and a bottom surface facing the output surface in parallel. The bottom surface is provided with a reflecting prism having a reflecting hollowed portion in a form of a wedge extending along the receiving end surface and a dividing flat portion in a form of a strip extending orthogonal to the reflecting hollowed portion in the form of a wedge to divide the reflecting hollowed portion in the form of a wedge.
The backlight thus configured has a dividing flat portion dividing a reflecting plane in the form of a wedge providing reflection. As such, a prism sheet or the like can be dispensed with and a light guide plate suffices to emanate light having an appropriate light distribution. Furthermore, if its output surface is provided with a collecting prism, an uneven portion in the form of a prism of the output surface can effectively collect light to achieve high level of brightness. Furthermore, light emanating from the light guide plate can have a spatial distribution in intensity adjusted from example by changing both the reflecting hollowed portion in the form of a wedge in depth and the reflecting hollowed portion in the form of a wedge in width and the flat portion in width to consider their respective distances from the receiving end surface. As such, the backlight can be configured of a reduced number of members and hence at low cost to provide spatially uniform distribution and appropriate light distribution. More specifically, the above effect is obtained by gradually increasing in depth the reflecting hollowed portion in the wedge to consider its distance from the receiving end surface and by gradually decreasing in width the dividing flat portion to consider its distance from the receiving end surface. Thus the above effect can be readily achieved.
Desirably in the backlight the output surface is provided with a collecting prism having an uneven portion in the form of a prism with a plurality of parallel ridges extending in a direction orthogonal to the receiving end surface.
The output surface with a collecting prism arranged therein can provide high level of brightness in addition to appropriate distribution of light provided via a reflecting prism.
Furthermore, the present backlight can have the dividing flat portion in the form of a strip having a width that varies depending on its distance from the receiving end surface.
If the reflecting hollowed portion in the form of a wedge is changed only in depth to provide emanating light having spatially uniform distribution, the hollowed portion in a vicinity of the receiving end surface is required to have as extremely minute a height as approximately several xcexcm. In contrast, if it is combined with a dividing flat portion of a strip having a variable width, then the reflecting hollowed portion in the vicinity of the receiving end surface can have a depth that is readily processed and the backlight can thus be readily manufactured. Furthermore, conventionally, as seen in a direction parallel to the receiving end surface, reflecting hollowed portions in the form of wedges are formed uniformly, having a single size, and in this direction emanating light can hardly be controlled to have uniform intensity. The dividing flat portion adjusted in width can provide uniformity in this direction.
Desirably in the present backlight the reflecting hollowed portion in the form of a wedge has a depth gradually increasing depending on a distance thereof from the receiving end surface.
In a light guide plate, light has the highest level of intensity around the receiving end surface and reduces in intensity as it is farther away from the receiving end surface. A reflecting hollowed portion in the form of a wedge greater in depth allows reflected light to have a higher level of intensity and hence light emanating through the output surface to have a higher level of intensity. As such, a reflecting hollowed portion in the form of a wedge that is increased in depth, as described above, allows emanating light to have a uniform distribution regardless of its distance from the receiving end surface.
Desirably in the present backlight the dividing flat portion in the form of a strip has a width gradually decreasing depending on a distance thereof from the receiving end surface.
A region corresponding to a dividing flat portion in the form of a strip is not provided with a reflecting hollowed portion in the form of a wedge. If the dividing flat portion in the form of a strip has a width decreasing depending on its distance from the receiving end surface, the reflecting hollowed portion in the form of a wedge accordingly gradually increases. As such, regardless of the distance from the receiving end surface, emanating light can have uniform intensity.
Desirably the present backlight includes the dividing flat portion in the form of a strip having a width of at least 10 xcexcm.
If a dividing flat portion has a width of no less than 10 xcexcm a hollowed portion in the form of a wedge can be processed without using a high-precision threading tool and the backlight can thus be readily manufactured. As such, there can be implemented a backlight at low cost that allows emanating light to have a spatially uniform distribution and provides an appropriate light distribution.
Desirably in the present backlight a flat portion having a width of at least 10 xcexcm is provided between the reflecting hollowed portion in the form of a wedge and the reflecting hollowed portion in the form of a wedge adjacent thereto.
If there exists a flat portion provided between hollowed portions and having a width of no less than 10 xcexcm, the hollowed portion can be processed without using a high-precision threading tool and the backlight can thus be readily manufactured. As such, there can be provided a backlight at low cost that emanates light having a spatially uniform distribution and an appropriate light distribution.
Desirably in the present backlight the flat portion between the hollowed portions is deeper in level than the dividing flat portion, as seen in a direction from the bottom plane towards an inner portion of the light guide plate.
A flat portion between hollowed portions is formed deeper in level than a dividing flat portion traversing in a direction in which a reflecting hollowed portion in the form of a wedge extends. As such, the hollowed portion is not required to be processed with high precision and the backlight can thus be readily manufactured. Thus, there can be provided a backlight at low cost that provides a spatially uniform distribution and an appropriate light distribution.
Desirably in the present backlight the reflecting hollowed portion in the form of a wedge has an oblique plane closer to the receiving end surface and having an angle xcex1 of 40xc2x0 to 50xc2x0 to the bottom surface.
The range of angle adopted as above reduces reflection at the uneven portion in the form of a prism of the output surface to emanate light highly efficiently with an appropriate light distribution. Thus, there can be provided a backlight at low cost that is highly efficient and also provides an appropriate light distribution.
Desirably in the present backlight the uneven portion in the form of a prism has a convexity with an apex angle xcex2 of at least 100xc2x0.
As such, the output surface has an uneven portion in the form of a prism that reduces reflection to allow light having an appropriate distribution to be emanated highly efficiently. Thus, there can be implemented a backlight at low cost that is highly efficient and provides a spatially uniform distribution and an appropriate light distribution.
Desirably in the present backlight the uneven portion in the form of a prism has an unevenness having a ridge and a trough adjacent thereto with a difference therebetween in level randomly varying as seen in a direction from the bottom surface toward the output plane.
Thus Moire fringes of the uneven portion in the form of a prism and a pixel of a liquid crystal panel can be prevented to implement a backlight at low cost that provides high visibility and appropriate light distribution.
Desirably in the present backlight the dividing flat portion has a width randomly varying depending on a distance thereof from the receiving end surface.
A dividing flat portion also randomly varying in width can prevent Moire fringes between a reflecting hollowed portion in the form of a wedge and a pixel of a liquid crystal panel. Thus, there can be implemented a backlight at low cost that provides high visibility and appropriate light distribution.
Desirably in the present backlight the dividing flat portion in the form of a strip is curved in arrangement.
A dividing flat portion in the form of a strip that is curved in arrangement can prevent Moire fringes between a reflecting hollowed portion in the form of a wedge and a pixel of a liquid crystal panel. Thus, there can be implemented a backlight at low cost that provides high visibility and appropriate light distribution.
Desirably in the present backlight the reflecting hollowed portion in the form of a wedge has an oblique surface closer to the receiving end surface and having an angle periodically altered.
An oblique surface having an angle varying periodically as described above allows emanating light to have a wide angle of emanation in a direction perpendicular to the receiving end surface to provide the emanating light having a flat light distribution. Thus there can be provided a backlight at low cost that provides high visibility and appropriate light distribution.
Desirably in the present backlight the reflecting hollowed portion in the form of a wedge has an oblique surface closer to the receiving end surface and extending in a direction along the receiving end surface, in a form of a portion of a circumference of a cylinder concave to distant from the receiving end surface.
The oblique surface corresponding to a cylindrical surface that is concaved to be farther away from the receiving end surface, allows emanating light to have a wide angle of emanation in a direction perpendicular to the receiving end surface to provide the emanating light having a flat light distribution. Thus, there can be implemented a backlight at low cost that provides high visibility and appropriate light distribution.
Furthermore, desirably the present backlight further includes a scattering plate adjacent to the output surface to scatter a component of light traveling in a direction along the receiving end surface.
With a reflecting surface arranged as above, light reflected by a prism structure of the output surface is reflected while its angle in a direction perpendicular to the receiving end surface is maintained. As such, light in appropriate light distribution is emanated highly efficiently. Thus, there can be provided a backlight at low cost that is highly efficient and also provides appropriate light distribution.
Furthermore, desirably the present backlight further includes a scattering plate adjacent to the output surface to scatter a component of light traveling in a direction in a surface orthogonal to a direction along the receiving end surface.
The above scattering plate thus arranged allows emanating light to have a flat light distribution in a direction perpendicular to the receiving end surface. As such, there can be provided a backlight at low cost that provides high visibility and appropriate light distribution.
Furthermore, desirably the present backlight further includes an optical element provided between the light source and the receiving end surface and having an uneven structure in a form of a prism with a convexity arranged closer to the receiving end surface.
The above optical element arranged as above allows a reflecting surface of the form of a hollowed wedge to emanate light having more appropriate light distribution. Thus, there can be provided a backlight at low cost that provides appropriate light distribution.
Desirably in the present backlight the light guide plate is divided into portions in a direction perpendicular to the receiving end surface corresponding to a direction in which light propagates through the light guide plate, the portions being coupled together.
Dividing a light guide plate, as described above, eliminates the necessity of processing the light guide plate to have a single large area when it is manufactured. As such, it can be readily manufactured. Thus, there can be provided a backlight at low cost that provides spatially uniform distribution and appropriate light distribution.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.